Documentation/git-prune.txt: document unpacked logic

Clarifies the git-prune man page, documenting that it only
prunes unpacked objects.

Signed-off-by: Chris Frey <cdfrey@foursquare.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-prune.txt

@@ -18,12 +18,15 @@ git-prune. See the section "NOTES", below.
 
 This runs `git-fsck --unreachable` using all the refs
 available in `$GIT_DIR/refs`, optionally with additional set of
-objects specified on the command line, and prunes all
+objects specified on the command line, and prunes all unpacked
 objects unreachable from any of these head objects from the object database.
 In addition, it
 prunes the unpacked objects that are also found in packs by
 running `git prune-packed`.
 
+Note that unreachable, packed objects will remain.  If this is
+not desired, see linkgit:git-repack[1].
+
 OPTIONS
 -------